The present invention relates to a process and a device for monitoring the corrosive, erosive and/or encrusting nature of a liquid with respect to a metallic installation, such as, for example, a pipeline or a heat exchanger.
In handling a liquid in a metallic installation, there is sometimes the risk that the latter will suffer corrosion, erosion and/or local encrusting which can sooner or later interfere with the proper functioning of the installation. It is difficult to predict the appearance and development of these simultaneous phenomena of corrosion, erosion and encrusting of metallic installations, because they depend on a large number of factors, particularly the nature of the liquid treated, the possible presence of solid matter suspended in the treated liquid, the temperature, velocity and degree of turbulence of the liquid in the vicinity of the wall of the installation, the nature of the metal of the installation, the shape of the installation and its surface condition, especially its roughness.
Thus, in the presence of hard water, progressive furring, or scale formation, is generally observed in such installations; this furring, or scaling, which is particularly severe in the case of hot water, leads to progressive blockage of the installations and to a decrease in the efficiency of the heat exchangers.
In the presence of corrosive water such as softened water, there is furthermore the danger that installations made of stainless steel or galvanized steel can undergo local corrosion which can sometimes lead to perforation of the wall of the installation.
If the treated liquids contain solid materials in suspension, a local erosion of the walls of the installation, or a sedimentation in the zones of the installation where the liquid undergoes a sudden pressure drop, such as in enlargements of pipelines, or in bends, is sometimes observed.
Erosion or sedimentation phenomena can in particular occur in installations through which viscous liquids, such as slurries, flow, or in evaporators/crystallizers such as those commonly used for the treatment of aqueous solutions of sodium hydroxide originating from sodium chloride brine electrolysis cells.
It is known to utilize gravimetric methods of measurement to monitor the corrosive or encrusting character of liquids circulating in installations; as disclosed for example in the periodical Materials Protection, October 1962, at pages 10 to 19 and 27. These known methods consist of periodically withdrawing from the liquid a probe which is normally immersed therein, descaling the probe so as to remove materials which may be encrusted thereon, and weighing these materials as well as the probe. A comparison of the weight of the probe itself before and after the test makes it possible to assess the corrosive nature of the liquid, while the weight of the materials encrusted on the probe during the test is a measure of the encrusting nature of the liquid. These known methods have the disadvantage of being slow and rather imprecise, and they are incapable of providing an instantaneous indication of the surface condition of an installation in which the liquid is being treated, stored or conveyed.
It has also been proposed to monitor the corrosive character of liquids circulating in metallic installations by measuring the variation with time of the electrical resistance of a probe dipped in the liquid. This is disclosed in the periodical Corrosion, published by the National Association of Corrosion Engineers, Volume 14, March 1958, at pages 155t to 158t. While this known process permits precise, instantaneous and continuous monitoring of the corrosive nature of the liquid, it does not, however, make it possible to monitor the formation of crusts or of sediments on the walls of the installation. Furthermore, it suffers from the disadvantage of requiring precalibrated, expensive and fragile probes, which have to be replaced periodically.
It has also been proposed to monitor the formation of deposits of crusts on a wall in contact with a liquid, for example the wall of a heat exchanger, by measuring the variation, with time, of the temperature of the wall by means of a thermocouple seated in the wall, as disclosed in the periodical Chemical Engineering Progress, July 1975, Volume 71, No. 7, at pages 66 to 72. However, this known method does not lend itself to monitoring the corrosive nature of the liquid. Furthermore, it suffers from the disadvantage that it is greatly subject to variations in temperature of the medium in contact with the wall.
U.S. Pat. No. 3,612,998 issued to B. G. Turner et al on Oct. 12, 1971, proposes a process for detecting corrosion brought about by an electrokinetic phenomenon caused by the passage of a liquid at high velocity near a metallic component, which process consists of measuring the electric current generated by the continuous dissolution of the metallic component in the liquid under the action of the electrokinetic phenomenon.
This known process has the disadvantage that it is only applicable to a particular type of corrosion caused by the flow of liquids at very high velocity. It is not capable of detecting any other type of corrosion such as, for example, that which is inherent in the corrosiveness of soft waters, or of detecting erosion or encrusting. It has the additional disadvantage that it is not responsive to the surface condition of the metallic component and consequently it does not make it possible to assess the effect of the corrosion on the metallic component.